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Understanding laser focus will help you reason about difficulties that you are having with cutting your material. Here is a diagram of a general lens mfor a laser cutter: 
As the diagram indicates, material surfaces that are placed above or below the focal point will have a larger cutting area than the potential minimum size. A larger area means the same energy is dispersed over a larger area, and therefore may not create as deep of a cut as you are expecting.
For thicker materials, the focus length problem can become more obvious:
Ramp Test
Figuring out your ideal focal length can either take some iteration, or you can do some tests to identify the best length for your material and cut. A quick focal length test can be done with any short (about 10cm) scrap of wood material and a wooden block. Place the short piece on the block to create a ramp:
Without disturbing the position of the material, use the focus guide to determine whether your material needs to be adjusted. You can estimate the size of the laser focus spot using the ramp, either in your mind by guesstimating the thickness of your material, or precisely by measuring the angle of your ramp:
